Tissue contact sensing with a multi electrode ablation catheter

ABSTRACT

A method and system for assessing electrode-tissue contact before the delivery of ablation energy. The system may include a control unit programmed to determine a difference between a maximum impedance magnitude at a low frequency for a given electrode and an absolute minimum impedance magnitude at the low frequency across all electrodes, determine a difference between a maximum impedance magnitude at a high frequency for a given electrode and an absolute minimum impedance magnitude at the high frequency across all electrodes, and determine a difference between a maximum impedance phase at the high frequency for a given electrode and an absolute minimum impedance phase at the high frequency across all electrodes. Differences may be correlated to one another using a linear model, the results determining electrode-tissue contact status. The results may be displayed in a graphical format for easy communication to the user.

CROSS-REFERENCE TO RELATED APPLICATION

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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FIELD OF THE INVENTION

The present invention relates to a method and system for assessingelectrode-tissue contact before the delivery of ablation energy.

BACKGROUND OF THE INVENTION

Many types of cardiac arrhythmia, conditions in which the heart's normalrhythm is disrupted, are often treated by ablation (for example, radiofrequency (RF) ablation, cryoablation, ultrasound ablation, laserablation, microwave ablation, and the like), either epicardially orendocardially. Cardiac ablation may be performed with a variety ofdevices, such as devices having expandable distal ends. For example,ablation of the tissue surrounding a pulmonary vein ostium may beperformed with a device having a distal end that is expandable into asubstantially circular configuration, such as the pulmonary veinablation catheter (PVAC®, Medtronic, Inc., Minneapolis, Minn.).

However, the success of any cardiac ablation procedure depends largelyon the quality of the lesion(s) created during the procedure. Lesionquality depends, in turn, on the quality of contact between the ablationelectrodes and the target tissue. Anatomical variations within thepulmonary veins, or other areas of target tissue, may cause a loss ofcontact between one or more electrodes and the target tissue.

Currently known methods for assessing contact between an electrode andtissue include visual contact assessment using fluoroscopic imaging.However, this method requires costly imaging equipment and can be timeconsuming. Other methods of electrode-tissue contact assessment involvemonitoring the temperature of and power delivered to each electrode, butsuch methods are typically used after the delivery of ablation energyhas already begun and often do not prevent unintended tissue damage.Additionally, these methods can be ineffective and subject to widevariation between patients.

As noted above, ablation may be applied via catheters designed todeliver amount of energy that optimize therapy yet minimize damage tosurrounding tissue. A catheter used to deliver electrical energy totissue has an inherent capacity of relaying electrical information to aremote impedance measurement device. If an impedance measurement is madepre- and post-ablation, an accurate assessment can be made as to thequality of the lesion. Therefore, the catheter's utility may be extendedbeyond its single role of delivering ablation into that of a lesiongauge, negating the need to introduce other gauges, cameras, or imagingsystems to perform the same functions.

Difficulties arise when attempting to make impedance measurements viacatheters, especially those with a high electrode count (for example,16-electrode catheters). For impedance measurements resolved by acatheter, it is necessary to discern a bipolar impedance existingbetween electrodes from a unipolar impedance existing from a singleelectrode to a neutral or patient return electrode. Currently knownsystems and methods do not perform impedance disambiguation, with nodiscernment between unipolar and bipolar impedance and separation ofthese from parasitic impedances for multi-electrode catheters. Thus, aphysician cannot attribute an impedance rending to the catheter, letalone to a specific location corresponding to catheter electrodes. As aresult, the physician cannot infer lesion quality. Therefore, in orderto imply lesion quality, the catheter and the impedance measurementsystem must “untangle” the multitude of impedance elements so as toprovide a clear indicator of a particular catheter electrode'sphysiological effect.

An important function must then be to sift and accurately resolve amultitude of unipolar and bipolar impedance elements resulting from themany circuitous pathways resulting from a multi-electrode catheterplaced inside the heart. Beyond the desired unipolar and bipolar resultsthat can be related to their respective electrodes, there are additionalundesired parasitic pathways that act to corrupt the desired impedancerenderings. These pathways must also be measured, and dissected, orde-embedded from the desired unipolar and bipolar readingsautomatically. Undesired parasitic pathways include catheter wire seriesimpedance, electric field (capacitive) coupling between catheter wires,and the placement of signal splitters and filters in-line with thecatheter that divert catheter sensed electrogram (EGM) signals for useby an electrocardiogram (ECG) monitor.

Because of the potentially long duration required to collect electricalinformation necessary to disambiguate, calculate, and report unipolarand bipolar impedance over a large set of catheter electrodes, thesystem must have the ability to perform impedance measurements rapidly.To enable rapid impedance measurements, the system must apply thatenergy across all catheter electrodes simultaneously so that measurementresults are correlated in time across frequency and electrode so thatresults are unaffected by slight movements of the catheter againsttissue within the heart. Additionally, the physician must have afunction that automatically scales the impedance measurement system'sdetector gain while maintaining the catheter electrodes' deliveredcurrent at safe, yet maximum, levels while the physician traverseswidely ranging tissue and fluid impedances. Widely disparate tissueimpedance could occur due to the difference of blood and heart tissue,ablated vs. non-ablated tissue, as well as from ice that forms anelectrically insulting barrier on the tip of a cryoablation catheter,assuming that an impedance measurement function is used to supportcryoablation lesion quality assessment.

Patient safety is a paramount concern, and delivery of measurementenergy must be low enough in amplitude so as to meet applicablestandards and possess a waveform that is biphasic so that inadvertentstimulation of cardio or nerve tissue does not occur. A furtherimportant consideration for an impedance measurement system is toprovide an automatic, traceable calibration conforming to applicablestandards so that the instrument's impedance renderings are trustworthy.

Finally, since the physician may use the impedance measurement functionto determine lesion quality (gauged by ablation impedance renderings)immediately prior to and after the moment of therapy, the impedancemeasurement system must remain connected in-line with the catheter andthe ablation generator simultaneously. Therefore, some mechanism mustsequence and protect the impedance measurement function during thedelivery of the high energy therapy.

It is therefore desired to provide a method and system for accurate andreliable electrode-tissue contact assessment, and for electrode-tissuecontact assessment before the delivery of ablation energy begins.

SUMMARY OF THE INVENTION

The present invention advantageously provides a method and system forassessing electrode-tissue contact before the delivery of ablationenergy. In one embodiment, the system may include a medical deviceincluding a treatment assembly having a plurality of electrodes and acontrol unit in communication with the medical device, the control unitprogrammed to: determine a difference value between a maximum impedancemagnitude at a low frequency for one of a plurality of electrodes and anabsolute minimum impedance magnitude at the low frequency across all ofthe plurality of electrodes; determine a difference value between amaximum impedance magnitude at a high frequency for the one of theplurality of electrodes and an absolute minimum impedance magnitude atthe high frequency across all of the plurality of electrodes; determinea difference value between a maximum impedance phase at the highfrequency for the one of the plurality of electrodes and an absoluteminimum impedance phase at the high frequency across all of theplurality of electrodes; correlate the difference values to each otherfor each of the plurality of electrodes; and assign to each of theplurality of electrodes a discrete prediction variable based on thecorrelations. The low frequency may between approximately 5 kHz andapproximately 15 kHz, such as 12.5 kHz, and the high frequency may bebetween approximately 80 kHz and approximately 140 kHz, such as 100 kHz.Correlating the differences to each other for each of the plurality ofelectrodes may include applying a linear model to the difference values.Further, each of the plurality of electrodes may be assigned a discreteprediction variable that is either 0 or 1. For example, a discreteprediction variable of 0 may be assigned to an electrode when theelectrode is not in contact with tissue and a discrete predictionvariable of 1 may be assigned to an electrode when the electrode is incontact with the tissue. Further, a discrete prediction value of 2 maybe assigned to an electrode when the electrode is in excessive contactwith tissue. Ablation energy may be delivered to any of the plurality ofelectrodes to which a discrete prediction variable of 1 is assigned.

In another embodiment, the system may include a medical device includinga treatment assembly having a plurality of electrodes and a control unitin communication with the medical device, the control unit programmedto: record a impedance magnitude value at a first frequency by each of aplurality of electrodes and determine a maximum impedance magnitudevalue of the plurality of impedance magnitude values at the firstfrequency; record an impedance magnitude value at a second frequency byeach of the plurality of electrodes and determine a maximum impedancemagnitude value of the plurality of impedance magnitude values at thesecond frequency; record an impedance phase value at the secondfrequency by each of the plurality of electrodes and determine a maximumimpedance phase value of the plurality of impedance phase values; recorda minimum impedance magnitude value at the first frequency across all ofthe plurality of electrodes; record a minimum impedance magnitude valueat the second frequency across all of the plurality of electrodes;recording a minimum impedance phase value at the second frequency acrossall of the plurality of electrodes; for each of the plurality ofelectrodes, calculate a first difference value between the maximumimpedance magnitude value and the minimum impedance magnitude valuerecorded at the first frequency; for each of the plurality ofelectrodes, calculate a second difference value between the maximumimpedance magnitude value and the minimum impedance magnitude valuerecorded at the second frequency; for each of the plurality ofelectrodes, calculate a third difference value between the maximumimpedance phase value and the minimum impedance phase value recorded atthe second frequency; and for each of the plurality of electrodes, applya linear model to all of the first, second, and third difference valuesto determine a continuous prediction value for each of the plurality ofelectrodes. The system may further include a display in communicationwith the control unit. A continuous prediction value for each of theplurality of electrodes may be determined using the following equation:continuous prediction value=a*1+b*ΔZ₁₀₀+c*ΔZ_(12.5)+d*ΔØ₁₀₀. The controlunit may be further programmed to assign a discrete prediction value toeach of the plurality of electrodes based on a corresponding continuousprediction value, and the display may be configured to display at leastone of the continuous prediction value for each of the plurality ofelectrodes and the discrete prediction value for each of the pluralityof electrodes. The continuous prediction value and/or the discreteprediction value may be displayed in text, color, and/or graphicalformats. For example, a graphical representation of the plurality ofelectrodes may be displayed, with each of the plurality of electrodesbeing displayed as a color that corresponds to at least one of thecontinuous prediction value and the discrete prediction value, thecolors changing in real time with the at least one continuous predictionvalue and discrete prediction value. The second frequency may be higherthan the first frequency. For example, the first frequency may bebetween approximately 5 kHz and approximately 15 kHz, such as 12.5 kHz,and the second frequency may be between approximately 80 kHz andapproximately 140 kHz, such as 100 kHz. The continuous prediction valuemay be a number between 0 and 1. Further, a discrete prediction value of0 may be assigned to a continuous prediction value that is less than acutoff value and a discrete prediction value of 1 may be assigned to acontinuous prediction value that is greater than or equal to the cutoffvalue. For example, the cutoff value may be 0.4 or 0.5. Further, thecontinuous prediction value may be a number that is equal to or greaterthan 0. A discrete prediction value of 0 may be assigned to a continuousprediction value that is less than a first cutoff value, a discreteprediction value of 1 may be assigned to a continuous prediction valuethat is greater than or equal to the first cutoff value and less than asecond cutoff value, and a discrete prediction value of 2 may beassigned to a continuous prediction value that is greater than or equalto the second cutoff value. For example, the first cutoff value may be0.4 and the second cutoff value may be 1.5

In another embodiment, the system may include a medical device includinga treatment assembly having a plurality of electrodes and a control unitin communication with the medical device, the control unit programmedto: record a first maximum impedance magnitude value from each of aplurality of electrodes at 12.5 kHz; record a second maximum impedancemagnitude value from each of the plurality of electrodes at 100 kHz;record a maximum impedance phase value from each of the plurality ofelectrodes at 100 kHz; record a first minimum impedance magnitude valueacross all electrodes of the plurality of electrodes at 12.5 kHz; recorda second minimum impedance magnitude value across all electrodes of theplurality of electrodes at 100 kHz; record a minimum impedance phasevalue across all electrodes of the plurality of electrodes at 100 kHz;determine a difference value between the first maximum impedancemagnitude value for each of the plurality of electrodes and the firstminimum impedance magnitude value; determine a difference value betweenthe second maximum impedance magnitude value for each of the pluralityof electrodes and the second minimum impedance magnitude value;determine a difference value between the maximum impedance phase valuefor each of the plurality of electrodes and the minimum impedance phasevalue; apply a linear model to the difference values to determine acontinuous prediction value between 0 and 1 for each of the plurality ofelectrodes; and assign a discrete prediction value of 0 to each of theplurality of electrodes for which the continuous prediction value isless than a cutoff value and assign a discrete prediction value of 1 toeach of the plurality of electrodes for which the continuous predictionvalue is equal to or greater than the cutoff value. The system mayfurther include a display in communication with the control unit, thedisplay configured to display at least one of the continuous predictionvalue for each of the plurality of electrodes and the discreteprediction value for each of the plurality of electrodes in at least oneof text, color, and graphical formats.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 shows an exemplary system for assessing electrode-tissue contact;

FIGS. 2A and 2B show exemplary distal portions of multi-electrodeablation catheters;

FIG. 3 shows a cross-sectional view of a treatment assembly of amulti-electrode ablation catheter in partial contact with target tissue;

FIGS. 4A-4D show a flowchart for a method of assessing electrode-tissuecontact;

FIGS. 5A and 5B show exemplary power and temperature measurements;

FIG. 6 shows a grid for qualifying electrode-tissue contactdeterminations;

FIG. 7 shows a summary chart of factor analysis;

FIG. 8 shows a schematic view of a multi-electrode device impedancemodel;

FIG. 9 shows a schematic view of a two-electrode device impedance model;

FIG. 10 shows a schematic view of a circuit implementation of amulti-frequency generator, Gaussian filtering, and summing M frequenciesinto one multi-spectral, constant current waveform without unipolarenergy;

FIGS. 11A and 11B show a schematic view of a circuit implementation of amulti-frequency generator, Gaussian filtering, summing M frequencies,constant current waveform amplifiers, and a catheter electrodemultiplexing system;

FIG. 12A shows a schematic view of a first exemplary calibration withCondition A, wherein parasitic series and shunt impedance elements aredetermined;

FIG. 12B shows a schematic view of a first exemplary calibration withCondition B, wherein parasitic series and shunt impedance elements aredetermined;

FIG. 13 shows a schematic view of a second exemplary calibration,wherein offsets are determined using traceable sources;

FIG. 14 shows a flow chart for gain adjustment;

FIG. 15 shows a graphical view of a two-frequency patient-deliveredcurrent waveform in frequency domain;

FIG. 16 shows a graphical view of a two-frequency patient-deliveredcurrent waveform in time domain; and

FIG. 17 shows a schematic view of an impedance meter isolation system.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1-3, an exemplary system for assessingelectrode-tissue contact is shown. The system 10 may generally include atreatment device, such as an ablation catheter 12, for thermallytreating an area of tissue and a console 14 that houses various systemcontrols. The system 10 may be adapted for the use of one or more energymodalities, such as radiofrequency (RF) ablation, cryoablation,electroporation, pulsed field ablation, and/or microwave ablation.

The ablation catheter 12 may include an elongate body 16 having a distalportion 18 and a proximal portion 20. The distal portion 18 may includea treatment assembly 22 with one or more ablation electrodes 24. Forexample, the treatment assembly 22 may include one or more carrier arms30 each bearing one or more ablation electrodes 24. Although not shown,one or more of the carrier arms 30 may also bear one or more mappingelectrodes, pacing electrodes, reference electrodes, radiopaque markers,sensors (such as temperature and/or pressure sensors), and/or othercomponents.

Non-limiting examples of treatment assemblies are shown in FIGS. 1-3. Asshown in FIG. 1, the treatment assembly 22 may include a shaft 32slidably and rotatably movable within the elongate body and having adistal tip 34. The treatment assembly may also include a flexiblecarrier arm 30 having a distal portion 36 and a proximal portion 38, thedistal portion 36 being coupled to the shaft distal tip 34 such thatlongitudinal movement and/or rotation of the shaft 32 within theelongate body 16 may adjust the configuration of the treatment assembly22. For example, advancement of the shaft 32 distally may cause thetreatment assembly 22 to have a linear or at least substantially linearconfiguration (not shown) that may be used for delivery of the device tothe target site, whereas retraction of the shaft 32 may cause thecarrier arm 30 to expand radially from the shaft 32 and assume an atleast substantially circular configuration. Further, the diameter of theat least substantially circular configuration of the carrier arm 30 maybe adjusted by rotation of the shaft 32 within the elongate body 16. Thecarrier arm 30 may bear a plurality of ablation electrodes 24 along itslength. The device shown in FIG. 1 may be similar to the PVAC ablationcatheter.

Alternatively, as shown in FIGS. 2A and 2B, the treatment assembly 22may include more than one carrier arm 30. For example, the treatmentassembly 22 may include three carrier arms 30 (as shown in FIG. 2A) orfour carrier arms (as shown in FIG. 2B). The carrier arms 30 may beradially arranged about the longitudinal axis 44 of the ablationcatheter, so as to create a symmetrical ablation pattern. Further, thetreatment assembly 22 may include a shaft 32 with a distal tip 34, towhich at least a portion of each carrier arm 30 is attached (as shown inFIG. 2A), with advancement and retraction of the shaft 32 transitioningthe treatment assembly 22 between a linear or at least substantiallylinear configuration (not shown) for delivery of the device to thetarget site and an expanded treatment configuration (as shown in FIG.2A). Alternatively, the carrier arms 30 may be coupled to each otherwithout a shaft 32 (as shown in FIG. 2B).

The carrier arms 30 shown in FIG. 2A may each have a sagittate shapewith the one or more ablation electrodes 24 being coupled to theproximal portion 46 of each carrier arm 30, so as to create a proximallyoriented ablation plane or face. Conversely, the ablation electrodes 24may be coupled to the distal portion 48 of each carrier arm 30, as shownin FIG. 2B, so as to create a distally oriented ablation plane or face.The device shown in FIGS. 2A and 2B may be similar to the multi-arrayseptal catheter (MASCO, Medtronic, Inc., Minneapolis, Minn.) and themulti-array ablation catheter (MAAC®, Medtronic, Inc., Minneapolis,Minn.), respectively. However, it will be understood that otherconfigurations other than those shown and described herein may be used,and that the treatment assembly may have any configuration, size, numberof electrodes, number of carrier arms, and/or other features that renderthe treatment assembly suitable for ablating tissue. For example, aballoon catheter with electrodes on an outer surface of the balloon or afocal catheter may also be used.

The console 14 may generally include an energy source, such as a RFenergy generator 52, one or more computers 54, and one or more otherexternal components or internal components such as an impedance meter55. The RF energy generator 52 may be in electrical communication withthe one or more ablation electrodes 24. The one or more computers 54 mayeach include one or more displays 56, processors 58, and/or user inputdevices, and the one or more processors 58 also may be in electricalcommunication with the one or more ablation electrodes 24. One or moreof the displays 56 and/or other user input devices may be wired to theconsole 14 or may be in wireless communication with the console 14.Optionally, the console 14 may also include one or more fluidreservoirs, valves, conduits, connectors, power sources, sensors, forcegauges, navigation systems, displays, speakers, and the like foradjusting and monitoring system parameters and/or for generating one ormore displays or alerts to notify the user of various system criteria ordeterminations.

When the multi-electrode treatment assembly 22 is placed proximate or atleast partially in contact with an area of tissue, one or more of theelectrodes 24 may be in contact with the tissue whereas one or more ofthe electrodes 24 may not. In the non-limiting example shown in FIG. 3,electrodes 23A and 24B are in contact with tissue whereas electrodes 24Cand 24D are not. Being able to determine before initiating energydelivery which electrode(s) 24 are not in contact with the tissue mayfacilitate safer, more effective, and faster ablation procedures, as theuser may be able to manipulate the catheter in real time to optimizecontact between tissue and as many of the electrodes as possible. Amethod for assessing electrode-tissue contact is shown in the flowchartof FIGS. 4A-4D.

Referring now to FIGS. 4A-4D, the three main parameters of interest forthis method are: (1) the difference between the maximum impedancemagnitude at a low frequency (for example, 12.5 kHz) for a givenelectrode and the absolute minimum impedance magnitude at the same lowfrequency across all electrodes for that ablation; (2) the differencebetween the maximum impedance magnitude at a high frequency (forexample, 100 kHz) for a given electrode and the absolute minimumimpedance magnitude at the same high frequency across all electrodes forthat ablation; and (3) the difference between the maximum impedancephase at a high frequency (for example, 100 kHz) for a given electrodeand the absolute minimum impedance phase at the same high frequencyacross all electrodes for that ablation. This method uses relativevalues instead of absolute values for contact assessment. Additionally,this method involves normalizing the impedance magnitude and phase databy the corresponding minimum value recorded during the procedure insteadof normalizing the impedance magnitude and phase data by thecorresponding maximum value. Since the catheter must be in blood at somepoint during the procedure and impedance values in blood are lower thanimpedance values when recorded by electrodes in contact with tissue, theminimum impedance values correspond to when the catheter is in blood.The minimum impedance value may be affected by various bloodcharacteristics, such as electrolyte, saline, and blood thinner levels.As determined experimentally, the average minimum (or “in blood”)impedance value is approximately 100.45 with a standard deviation ofapproximately 11.38, with a maximum value being approximately 126.6 anda minimum value being approximately 85.5. The maximum impedance valuesmay be obtained by sampling the impedance several times per second and“keeping” the maximum impedance value recorded over a one to two secondwindow.

In the first step 64, the treatment assembly 22 of the ablation catheter12 may be positioned within the blood at a location near but not incontact with the target site. For example, if the target site is apulmonary vein ostium, the treatment assembly 22 may be positionedwithin the blood in the left atrium of the patient's heart. For eachelectrode 24, three impedance measurements may be acquired. Further,these measurements may be acquired at a first low frequency and a secondhigh frequency. As shown in FIG. 4A, a first impedance magnitude may berecorded at a low frequency (that is, a low-frequency current may bepassed between two electrodes) and a second impedance magnitude may berecorded at a high frequency (that is, a high-frequency current may bepassed between two electrodes) while the electrodes 24 are in blood andnot in contact with tissue. As referred to throughout, the low frequencymay be in the range of approximately 5 kHz to approximately 15 kHz (forexample, 12.5 kHz) and the high frequency may be in the range ofapproximately 80 kHz to approximately 140 kHz (for example, 100 kHz).Finally, an impedance phase may be recorded at the same high frequencywhile the electrodes 24 are in blood and not in contact with tissue.These three measurements are recorded for each electrode 24, andalthough shown as being a sequential process in FIG. 4A, measurementsmay be taken from all electrodes simultaneously or sequentially.Additionally, the measurements at different frequencies may be takensequentially simultaneously. Values may be measured and calculations maybe updated every ⅛ second to 1 second. As a non-limiting example,impedance magnitude and phase may be measured simultaneously from pairsof electrodes, but the measurements from electrode pairs may be takensequentially. Impedance values recorded at 12.5 kHz may measureextracellular fluid resistance, whereas impedance values recorded at 100kHz may measure resistance of intracellular fluid and membranes.

In the second step 66 shown in FIG. 4A, a first minimum impedancemagnitude may be recorded at a low frequency, such as 12.5 kHz, acrossall electrodes 24 and a second minimum impedance magnitude may berecorded at a high frequency, such as 100 kHz, across all electrodes 24while the electrodes 24 are still in blood and not in contact withtissue. Finally, an impedance phase may be recorded at the same highfrequency (100 kHz) across all electrodes 24 while the electrodes 24 arestill in blood and not in contact with tissue. For example, the systemmay continuously record impedance magnitude values when the lowfrequency energy is delivered and may continuous record impedancemagnitude and phase values when the high frequency energy is delivered.The one or more processors 58 may store a minimum magnitude value at thelow frequency, a minimum magnitude value at the high frequency, and aminimum phase value at the high frequency. If a new minimum value isrecorded for any of the values of interest, the new minimum valuereplaces the previous minimum value as the stored minimum value.Further, the minimum value may be a single discrete value or it may bethe average or median of a plurality of minimum values in order toremove outlier values.

In the third step 68 shown in FIG. 4B, the treatment assembly 22 may bepositioned so that it is at least partially in contact with an area oftissue (for example, the target tissue). Once the treatment assembly 22is at least partially in contact with the target tissue, three moremeasurements may be taken for each electrode 24. Alternatively,measurements may be continually measured while positioning the treatmentassembly 22 (and updated every ⅛ second to 1 second) to assist in theplacement of the treatment assembly 22 in contact with tissue. A firstimpedance magnitude may be recorded at a low frequency, such as 12.5kHz, and a second impedance magnitude may be recorded at a highfrequency, such as 100 kHz, while the treatment assembly 22 is at leastpartially in contact with tissue. Finally, an impedance phase may berecorded at the same high frequency (100 kHz) while the electrodes 24are in blood and not in contact with tissue. These three measurementsare recorded for each electrode 24, and although shown as being asequential process in FIG. 4B, measurements may be taken from allelectrodes simultaneously or sequentially.

In the fourth step 70 shown in FIG. 4C, a first minimum impedancemagnitude may be recorded at a low frequency, such as 12.5 kHz, acrossall electrodes 24 and a second minimum impedance magnitude may berecorded at a high frequency, such as 100 kHz, across all electrodes 24while the treatment assembly 22 is at least partially in contact withtissue. Finally, an impedance phase may be recorded at the same highfrequency (100 kHz) across all electrodes 24 while the treatmentassembly 22 is at least partially in contact with tissue.

In the fifth step 72 shown in FIG. 4C, each of the measurements recordedin the fourth step 70 may be compared to the measurements taken in thefirst 64, second 66, and third 68 steps. For example, it may bedetermined whether the minimum impedance magnitude recorded at the lowfrequency (12.5 kHz) across all electrodes in the fourth step 70 is lessthan the “in blood” impedance magnitudes of each electrode 24 (of thefirst step 64) and all previously recorded minimum impedance magnitudes(in the second step 66). This determination may be referred to as “BoxA.” Further, it may be determined whether the minimum impedancemagnitude recorded at the high frequency (100 kHz) across all electrodesin the fourth step 70 is less than the “in blood” impedance magnitudesof each electrode 24 (of the first step 64) and all previously recordedminimum impedance magnitudes (in the second step 66). This determinationmay be referred to as “Box B.” If the answer to the Box A determinationis “yes,” the Box A value is the minimum impedance magnitude recorded at12.5 kHz, or Z_(min,12.5). If the answer to the Box A determination is“no,” the Box A value is the smallest previously recorded Box A value or“in blood” value, whichever is smaller. Likewise, if the answer to theBox B determination is “yes,” the Box B value is the minimum impedancemagnitude recorded at 100 kHz, or Z_(min,100). If the answer to the BoxB determination is “no,” the Box B value is the smallest previouslyrecorded Box B value or “in blood” value, whichever is smaller.

Additionally, it may be determined whether the minimum impedance phaserecorded at the same high frequency (100 kHz) across all electrodes inthe fourth step 70 is less than the “in blood” impedance phase of eachelectrode 24 (of the first step 64) and all previously recorded minimumimpedance magnitudes (in the second step 66). This determination may bereferred to as “Box C.” If the answer to the Box C determination is“yes,” the Box C value is the minimum impedance phase recorded at 100kHz, or Ø_(min,100). If the answer to the Box C determination is “no,”the Box C value is the smallest previously recorded Box C value or “inblood” value, whichever is smaller.

In the sixth step 74 shown in FIG. 4D, the impedance magnitude recordedin the third step 68 at the low frequency (12.5 kHz) for each electrode24 may be referred to as “Box D,” the impedance magnitude recorded inthe third step 68 at the high frequency (100 kHz) for each electrode 24may be referred to as “Box E,” and the impedance phase recorded in thethird step 68 at the same high frequency (100 kHz) for each electrode 24may be referred to as “Box F.” The change in impedance magnitude may becalculated for each of Boxes D and E and the change in impedance phasemay be calculated for Box F. For example, the difference in impedancemagnitude at the low frequency (12.5 kHz) may be the Box D value and theminimum impedance recorded at the low frequency (12.5 kHz) in Box A (asshown in Equation (1) below), the difference in impedance magnitude atthe high frequency (100 kHz) may be the Box E value and the minimumimpedance recorded at the high frequency (100 kHz) in Box B (as shown inEquation (2) below), and the difference in impedance phase at the highfrequency (100 kHz) may be the Box F value and the minimum impedancephase recoded at the high frequency (100 kHz) in Box C, as shown inEquation (3) below:

ΔZ _(12.5)=BOX D Value−Z _(min,12.5)  (1)

ΔZ ₁₀₀=BOX E Value−Z _(min,100)  (2)

ΔØ₁₀₀=BOX F Value−Ø_(min,100)  (3)

In the seventh step 76 shown in FIG. 4D, a linear model may be appliedfor each electrode 24 according to Equation (4) below:

[Result]=a*1+b*ΔZ ₁₀₀ +c*ΔZ _(12.5) +d*ΔØ ₁₀₀   (4)

with “a,” “b,” “c,” and “d” being coefficients (for example, thecoefficients “a,” “b,” “c,” and “d” may be based on clinical data). Theresult is a continuous number (which may be referred to herein as a“continuous prediction value”) that may then be compared to a cutoffvalue to determine whether the electrode-tissue contact status is “nocontact” (and therefore assigned a value of 0) or “good contact” (andtherefore assigned a value of 1). To fit the linear model to the data,the status of “contact” may be given a value of 1 and a status of “nocontact” may be given a value of 0. Then, the coefficients may becalculated that best fit the development data. For example, analysis ofthe development data may determine the values of coefficients “a”, “b”,“c”, and “d” that best fit the model to predict contact (1) or nocontact (0). The AZ values may be calculated by subtracting the currentZ value from the minimum measured Z value. The Z value may be either themagnitude of the impedance or the real component of the impedance. Thephase value may be either in the form of the phase in degrees or theimaginary component of the impedance. Additionally,

A status of “contact,” regardless of the type of contact, may be groupedtogether. For example, an electrode may have a status of “contact” whenthe electrode in optimal contact with tissue and when the electrode isin excessive contact with the tissue (such as when the electrode isburied in the tissue). In addition to a value of “contact” or “nocontact,” However, the model may be further developed to give acontinuous result value between 0 and 2, in which case the contactstatus identification could be expanded to include “no contact,”“contact,” or “excessive contact.” The coefficients may be calculated inthe same way as discussed above for the “contact”/“no contact”evaluation. As a non-limiting example, the electrode-tissue contactstatus may be determined to be “no contact” when the continuousprediction value is less than or equal to a first cutoff value (forexample, 0.4 or 0.5), “contact” when the continuous prediction value isgreater than the first cutoff value but less than or equal to a secondcutoff value (for example, 1.5), and “excessive contact” when thecontinuous prediction value is greater than the second cutoff value.Further, a status of “no contact” may be assigned a discrete predictionvalue of 0, a status of “contact” may be assigned a discrete predictionvalue of 1, and a status of “excessive contact” may be assigned adiscrete prediction value of 2.

Still further, it will be understood that the model may be furtherdeveloped to include more than three contact status categories. As anon-limiting example, the model may be developed to provide fordeterminations of “no contact” (for example, a continuous predictionvalue less than or equal to a first cutoff value), “previously ablatedtissue” (for example, a continuous prediction value greater than thefirst cutoff value but less than or equal to a second cutoff value),“contact” (for example, a continuous prediction value greater than thesecond cutoff value but less than or equal to a third cutoff value), and“excessive contact (for example, a continuous prediction value greaterthan the third cutoff value but greater than or equal to a fourth cutoffvalue). In this case, a status of “no contact” may be assigned adiscrete prediction value of 0, a status of “previously ablated tissue”may be assigned a discrete prediction value of 1, a status of “contact”may be assigned a discrete prediction value of 2, and a status of“excessive contact” may be assigned a discrete prediction value of 3.

The confidence interval of the linear model results may then becalculated and then displayed to the user. For example, the system mayhave collected enough data for the algorithm to return results with a95% confidence interval. However, higher or lower confidence intervalsmay also be displayed, based on the amount of measurements collected. Inaddition to or instead of displaying the discrete 0 or 1 (or 2, 3, etc.)result to the user, the linear prediction continuous result (thatcontinuous prediction value) may be displayed to the user in real time.Displaying the continuous result value may be useful, for example, toassess a state of intermittent contact for one or more electrodes, andto evaluate contact continuously during electrode placement and thetreatment procedure. Values may be displayed to the user as text, in baror other graphical formats, in a continuously changing linear display,by color, or other means. As a non-limiting example, a graphicalrepresentation of the treatment assembly 22 with one or more electrodes24 may be displayed to the user. As the treatment assembly 22 ispositioned or repositioned at or proximate a target tissue site, anumerical value, graphical depiction, color value, or othercharacteristic associated with each electrode 24 may change in real timeso the user can easily visualize the changes in contact status of eachelectrode. For example, the color of each area o of the graphicalrepresentation corresponding to an electrode 24 may change color in realtime, with red representing a status of “no contact” or “excessivecontact” and green representing a status of “contact.” Further, as acontinuous prediction value approaches a cutoff value, the colorassociated with that electrode may change to a color on the spectrumbetween the color associated with the color of the next cutoff value.Alternatively, discrete or continuous prediction values may be presentedto the user in a graphical representation of a gauge, with a needlemoving between values. Although the system 10 may display to the user astatus of “contact” or “no contact” (or other status, such as “excessivecontact”) in text based on the algorithm results discussed above, thesystem may additionally or alternatively display a numerical result tothe user and the user can then determine, for example, when contact issufficient or excessive without relying on the system's determination.Still further, the system 10 may automatically deactivate or turn off,or display a suggestion to the user that one or more electrodes bedeactivated or turned off, if the contact status for those one or moreelectrodes is, for example, “no contact,” “intermittent contact,” or“excessive contact.” Similarly, the system 10 may automatically (orprompt a user input to) extend the ablation time for one or moreelectrodes if the system 10 determines that those one or more electrodeshave intermittent contact with the target tissue. It will be understoodthat the data discussed herein, including continuous prediction valuesand discrete prediction values, may be presented to the user in any of avariety of ways.

If the required time has elapsed between impedance measurements and theprocedure is considered to be over, the procedure ends (for example, byceasing delivery of ablation energy to the electrodes 24). If therequired time has elapsed between the impedance measurements has elapsedbut the procedure is not considered to be over, the method may berepeated from the third step 68 shown in FIG. 4B. The method ofassessing electrode-tissue contact disclosed herein may be usedthroughout a procedure, between periods of delivery of ablation energy.During periods of ablation energy delivery, parameters such astemperature and power may be used to assess electrode-tissue contact.Further, ablation energy may be delivered to any of the electrodes 24that are determined to be in contact with an area of target tissueand/or the treatment assembly 22 may be repositioned until more or allof the electrodes 24 are in contact with the area of target tissue.

Although the method includes recording impedance measurements before andafter electrode contact with tissue, impedance measurements arecontinuously recorded in real time as the catheter is being positioned.For example, when the electrodes are located in the blood and not incontact with tissue, this contact status will be communicated in realtime to the user. Additionally or alternatively, recoded impedancevalues may be communicated in real time to the user without previouscontact status interpretation. As the electrodes come into contact withtissue, the impedance measurements for some or all of the electrodes maychange, which will also be communicated to the user in real time ascontact is established. As a non-limiting example, impedancemeasurements may be recorded from each electrode multiple times persecond (for example, approximately eight times per second) and thenaveraged for display to the user. Additionally, impedance measurements(phase and magnitude) may be recorded for both the high and lowfrequencies simultaneously.

Exemplary Test Data

An initial pool of 180 data points was collected. Each data point wasfrom one electrode of a nine-electrode treatment assembly over 20ablation procedures. In order to normalize the measurements, a minimumimpedance magnitude and minimum impedance phase were measured from anelectrode that was confirmed to not be touching tissue (that is, theelectrode was located within the left atrium, away from the atrialwalls). Each electrode was then assigned a value of 0 (“no contact”) or1 (“good contact”). As an electrode temperature will not increase duringablation when the electrode is not in contact with tissue because of thecooling effect of surrounding blood flow, temperature measurementsobtained during ablation were used as an adjudication method. MATLAB wasused to fit a least-squares regression curve using the assigned contactstate of each electrode as a discrete response variable and thenormalized impedance measurements were used as continuous predictionvalues. Several polynomial models were used as regression curve-fittingmethods to determine effects on algorithm performance. Further, theleast-squares fit result (a continuous value between 0 and 1) wasrounded to the nearest integer (0 or 1) to determine the algorithm'scontact result (that is, “no contact” or “good contact”). If thealgorithm result was greater than or equal to the cutoff value, theresult was rounded to 1. Conversely, if the algorithm result was lessthan the cutoff value, the result was rounded to 0. Additional contactstatus qualifiers may be used, such as “excessive contact” and“previously ablated tissue,” as discussed above. In such cases,additional cutoff values may be used. Decreasing the cutoff value raisedthe algorithm's sensitivity and may lower the specificity due to aprobable increase in the number of false positives and decreased in thenumber of false negatives. Increasing the cutoff value had the oppositeeffect. For example, it was determined that a cutoff value of 0.4 wasbetter at identifying when electrodes were in good contact with tissue,whereas an algorithm with a cutoff value of 0.5 was better atidentifying when electrodes were not in contact with tissue. Inpractice, however, the cutoff value may be set according to userpreferences with respect to high/low sensitivity and specificity.

The 180 data points were then separated into two groups: a developmentgroup including 120 selected data points of the 180 initial data pointsused to develop the algorithm, and a test group including the remaining60 data points of the 180 initial data points, to which test group thealgorithm was applied. The 120 selected data points includedapproximately two-thirds of the data points defined as “no contact”(that is, a having a value of 0) and approximately two-thirds of thedata points defined as “good contact/low power” (that is, having a valueof 1). The remaining approximately one-third of the data points definedas “no contact” and approximately one-third of the data points definedas “good contact/low power” were included in the test group. Thedevelopment group data were used to develop and fine-tune the predictionmethod, whereas the performance of the prediction method was tested onthe test group data.

For the development group, the three main parameters of interest(discussed regarding FIGS. 4A-4D) were used to determine the optimumlinear combination using the MATLAB® (The Mathworks, Inc., Natick,Mass.) function “fitlm” (least squares fit of the response to the data).Multiple linear combination methods were used. The response variable ofthe linear combination was the group number (0 for “no contact” and 1for “good contact”). Since the input variables were continuous but theresponse variables were discrete, the result from the linear combinationwas rounded to the nearest integer. The rounded result from the linearcombination was then compared with the expected result (the group numberdetermined from inspection of the plots and mean temperature and powerdata).

To assess algorithm performance, the sensitivity, specificity andpositive predict values (PPV) for each curve-fitting method werecalculated using the following equations:

Sensitivity=Number of True Positives÷(Number of True Positives+Number ofFalse Negatives)  (5)

Specificity=Number of True Negatives÷(Number of True Negatives+Number ofFalse Positives)  (6)

Positive Predictive Value=Number of True Positives÷(Number of TruePositives+Number of False Positives)  (7)

Negative Predictive Value=Number of True Negatives÷(Number of TrueNegatives+Number of False Negatives)  (8)

As shown in FIG. 6, the term “True Positive” used above refers to a casein which the algorithm assigns “good contact” when the electrode reallyis in contact with tissue. The term “True Negative” used above refers toa case in which the algorithm assigns “no contact” when the electrodereally is not touching tissue. The term “False Positive” used aboverefers to a case in which the algorithm assigns “good contact” when theelectrode really is not touching tissue. Finally, the term “FalseNegative” used above refers to a case in which the algorithm assigns “nocontact” when the electrode really is in contact with tissue.

For the test group, the same three main parameters of interest(discussed regarding FIGS. 4A-4D) were used to determine the optimumlinear combination. Linear combinations created from the developmentgroup data were applied to the test group data. The response variable ofthe linear combination was the group number (0 for “no contact,” 1 for“good contact”). Again, since the input variables were continuous butthe response variable was discrete, the result from the linearcombination was rounded to the nearest integer. The rounded result fromthe linear combination was then compared with the expected result (thegroup number determined from inspection of the plots and meantemperature and power data). The sensitivity, specificity, positiveprediction value, and negative prediction value were calculated for eachlinear combination method.

The linear models used in both the development group and the test groupwere the linear model (linear terms for each predictor), interactionsmodel (products of pairs of predictors with no squared terms), purequadratic model (linear terms and squared terms), quadratic (productterms and squared terms), and polyijk model (polynomial with all termsup to degree I in the first predictor, degree j in the second predictor,etc.). All of these models are basic linear models available for use inMATLAB, and experimental data showed that the linear model type does notsignificantly impact the statistical results. The simplest curve-fittingmethod, a linear method, achieved performance comparable with morecomplex methods. The linear method algorithm achieved a sensitivity of93%, a specificity of 85%, and a PPV of 92% for the development groupand a sensitivity of 97%, a specificity of 90%, and a PPV of 92% for thetest group.

Using pre-ablation predictors to create the algorithm may be moresensitive than using post-ablation predictors. Further, using onlypre-ablation predictors may be better than using a combination ofpre-ablation predictors and post-ablation predictors. However, thismethod and algorithm still performs well on previously ablated tissue.The algorithm may be used to distinguish between unablated tissue,previously ablated tissue, and no contact. For example, a lowercontinuous prediction value may indicate no contact, a higher continuousprediction value may indicate previously ablated tissue, and a highcontinuous prediction value may indicate unablated tissue.

Two data adjudication methods were used to determine if each electrodein the data set was in good contact with tissue or not in contact withtissue: (1) temperature-power method used to adjudicate non-superiorvena cava (non-SVC) ablations, and (2) histology to adjudicate SVCablations. In the first method, temperature and power measurements wereused to determine electrode-tissue contact based on the conditionsdescribed in Table 1.

TABLE 1 Assessment of electrode contact. Category Mean Temperature (°C.) Mean Power (W) Good Contact ≧47 ≧3 Low Power ≧47 <3 No Contact <47N/AElectrodes in the “low power” category were defined to be in good tissuecontact. However, if the third electrode-tissue contact qualifier“excessive contact” is used, the “low power” results may be consideredto be indicated of excessive contact between an electrode and tissue. Anon-limiting example of temperature and power measurements is shown inFIGS. 5A and 5B. FIG. 5A shows examples of “no contact,” “good contact,”and “low power,” whereas FIG. 5B shows examples of “no contact,” “goodcontact,” and “intermittent contact.” Intermittent contact may beindicated by impedance values that fluctuate between low impedancevalues and high impedance values (as shown for electrodes 1, 4, 7, and 9in FIG. 5B).

In the second method, histology was examined following an ablationprocedure. Histology data was available from chronic porcine studies.All ablations performed in the chronic studies were superior vena cava(SVC) ablations. All SVC ablations were assigned either “good contact”or “no contact” based on the histology results for each electrode. Itwas determined that there was no significant difference in algorithmperformance when an algorithm developed using a data set containing dataadjudicated using both methods was applied to a test group versus a testgroup containing only histology-adjudicated data. Therefore, therobustness of the temperature-power adjudication method is comparable tothe robustness of the histology adjudication method.

It was determined that algorithm performance was dependent upon thecomposition of the data set used to develop it. Using impedance datafrom SVC ablations where there was good contact and impedance data fromwhen the catheter was in blood from non-SVC ablations yielded superioralgorithm performance compared to when SVC and non-SVC ablations wereused. This may be because there was a clear separation between the “goodcontact” and “no contact” groups and the grey area was almost completelyeliminated. However, it was also determined that the composition of thetest group data set did not significantly impact algorithm results. Theresults obtained when the test group was comprised of only SVC ablationswas comparable to when the test group was comprised of both SVC andnon-SVC ablations. These findings are summarized in the chart shown inFIG. 7.

Although not expressly discussed herein, it will be understood that thealgorithm may also be adapted to accurately recognize when an electrodeis in contact with previously ablated tissue and indicate contact in amore continuous scale (for example, to indicate when there is alikelihood of good contact or excessive contact). Further, it will beunderstood that at least one processor 58 may include the algorithm(that is, may perform calculations using the algorithm) based onmeasurements received from the one or more electrodes 24 and/or fromother system components. The processor 58 may then communicate theresults of the calculations to the user via the one or more displays 56or other system components.

Exemplary System Configuration

The device, system, and method disclosed herein may be specifically usedto assess electrode-tissue contact using impedance measurements.However, the device, system, and method may also generally be used toprovide intracardiac multi-frequency, multi-electrode impedancemeasurements for other purposes. Given an N-electrode catheter, a methodand system is provided for resolving 2N total impedance elements: Nbipolar and N unipolar impedance elements, which result from theapplication of an N-electrode catheter in contact with blood,pre-ablation cardiac tissue, post-ablation cardiac tissue, or iceformation on a cryoablation catheter. The method and system may maximizea catheter delivered current that is consistent with applicable patientsafety standards and may minimize the risk of inadvertent cardiac ornerve stimulation by eliminating unipolar charge delivery. Further, themethod and system may distribute signals to the catheter electrodes viaa 2N switch array system. These functions may be performed whiledelivering a multi-spectral, simultaneous measurement current thatallows for the simultaneous measurement and resolution of unipolar andbipolar impedances.

The method and system may also be used for: (a) the measurement,detection, and resolution of cathode-delivered multi-spectral,simultaneous measurement currents into their mono-frequencyconstituents; (b) the disambiguation of unipolar and bipolar impedancesfrom parasitic pathway impedances resulting from non-ideal catheterdelivery wire impedance, inter-catheter wire field coupling mechanisms,and ablation delivery system signal splitter and filter conductivepathways and field coupling mechanisms; (c) the adaptation ofwide-ranging voltage and current measurements resulting from similarlywide-ranged values of blood, pre-ablation cardiac tissue, post-ablationcardiac tissue, or ice formation on a cryoablation catheter; (d) amethod of calibration of the impedance renderings that provides suretyto traceable international standards; and (e) the connection andsynchronization of the impedance measurement apparatus to aradiofrequency or pulsed field ablation generator 52.

Referring now to FIGS. 8-17, the particulars of an exemplary system for,generally, rendering alternating current impedances from a set ofablation catheter electrodes in contact with tissue and bodily fluid andfor, specifically, the assessment of electrode-tissue contact, areshown. The ablation catheter 12 having one or more electrodes 24 (asshown and described above) may be organized as a matrix of electricalnodes and bipolar impedances that reside between the electrodes andunipolar impedances that connect from the electrode and form a circuitthrough tissue to a neutral connection, usually referred to as thepatient return electrode, and finally returning to the instrument. Thesystem 10 may be used to resolve 2N total impedance elements: N bipolarand N unipolar impedance elements, which result from the application ofan N electrode catheter in contact with blood, pre-ablation cardiactissue, post-ablation cardiac tissue, and/or ice formation on acryoablation catheter.

Referring to FIGS. 8 and 9, a set of linear equations ensue viaapplication of Kirchoff's voltage law. Given N electrodes, there will beN unipolar and N bipolar impedance elements. The system of linearequations to solve the unipolar and bipolar impedances must then be:

Z=2*2N−1  (9)

The solution to this system first may be solved analytically, leaving aset of formulae in a microcontroller that only require the voltage andcurrent data from a set of analog-to-digital converters (ACDs). Thereare 2N ACDs: one to measure the source voltage V_(n), and a second toinfer the current formed by a voltage V_(R,n), residing across a knownresistance R_(n).

As shown in FIG. 8, a catheter 12 may include N electrodes (depicted asE₁, E₂, E₃ . . . E_(N)). Bipolar impedance Z_(b, n) may exist betweenelectrodes E₁ and E₂, bipolar impedance Z_(b, n+1) may exist betweenelectrodes E₂ and E₃, bipolar impedance Z_(b,N-1) may exist betweenelectrode E_(N) and the next lowest numbered electrode (depending on howmany electrodes 24 are included on the catheter 12). Finally, bipolarimpedance Z_(b,N) may exist between electrodes E_(N) and E₁. Further,unipolar impedance Z_(u,n) may exist between electrode E₁ and a patientreturn electrode (electrode not shown; pathway shown in FIG. 8 asV_(pre)), unipolar impedance Z_(u,n+1) may exist between electrode E₂and the patient return electrode (V_(pre)), unipolar impedance Z_(u,n+2)may exist between electrode E₃ and the patient return electrode(V_(pre)), and unipolar impedance Z_(u,N) may exist between electrodeE_(N) and the patient return electrode (V_(pre)). The circuit may alsoinclude parasitic impedance elements, depicted in FIG. 8 as Z_(serp,n),Z_(shp,n), Z_(serp,n+1), Z_(shp,n+1), Z_(serp,n+2), Z_(shp,n+2),Z_(serp,N), and Z_(shp,N).

Given this system of N electrodes, the number of unipolar and bipolarimpedance elements is equal to two times the number of electrodes. Thatis:

Z=2*2N−1  (9)

The number of unknowns and equations required to solve to calculate allthe unipolar and bipolar impedances is:

Equations=2*Z−1  (10)

In the circuit shown in FIG. 8, the known values may be R_(n) . . .R_(N) (resistance values), Z_(par,n) . . . Z_(par,N) (the parasiticimpedances), and V_(n) . . . V_(N) (voltage values at the nodes), andV_(Rn) . . . V_(RN) (voltage values across the resistors). The unknownvalues may be I_(b,n) . . . I_(b,N) (current values), Z_(u,n) . . .Z_(u,N) (unipolar impedance values), and Z_(b,n) . . . Z_(b,N) (bipolarimpedance values).

In the simplified example shown in FIG. 9, the catheter 12 includes twoelectrodes, E₁ and E₂ and a patient return electrode (not shown).Similar to what is shown in FIG. 8, a bipolar impedance Z_(b,1) mayexist between electrodes E₁ and E₂, unipolar impedance Z_(u,1) may existbetween electrode E₁ and the patient return electrode, and unipolarimpedance Z_(u,2) may exist between electrode E₂ and the patient returnelectrode. Parasitic impedance values Z_(par,n), Z_(shp,n), Z_(par,n+1),Z_(shp,n+1) may also exist in the circuit (the abbreviation “shp”represents a shunt path). In this example, there are two electrodes 24,and three unknowns (Z_(u,1), Z_(u,2), and Z_(b,1)). So, N=2 electrodesand unknowns=3. The number of impedance elements is calculated as:

Z=2*N−1=3  (11)

The number of equations to solve is calculated as:

Equations=2*(Z)−1=5  (12)

A first measurement may be conducted that provides equations that areinsufficient to the number of unknowns. To supply the remainingequations for a solution, a second condition may be applied thatannihilates the current in a system pathway, rendering another set ofequations that exceed the quantity of new unknowns. In combining theequations and unknowns from the first and second condition, equationsand unknowns are balanced, and all of the desired unipolar and bipolarimpedance elements can be found one. For example, three of the fiveequations to be solved may be solved with a first condition applied:

kvl _(I1) :V ₁ =I ₁(R ₁ +Z _(par1) +Z _(u,1))+I _(b,1) Z _(u,1)   (13)

kvl _(I2) :V ₂ =I ₂(R ₂ +Z _(par2) +Z _(u,2))+I _(b,1) Z _(u,2)   (14)

kvl _(Ib,1)=0=I _(b,1)(Z _(u,1) +Z _(u,2) +Z _(b,1))+I ₁ Z _(u,1) −I ₂ Z_(u,2)  (15)

For the final two equations, a second condition may be applied, whereinI₁≠0 and I₂=0:

kvl _(I1′) :V _(1′) =I _(1′)(R ₁ +Z _(par1) +Z _(u,1))+I _(b,1′) Z_(u,1)  (16)

kvl _(Ib,1′):0=I _(b,1′)(Z _(u,1) +Z _(u,2) +Z _(b,1))+I ₁ ′Z _(u,1)  (17)

The abbreviation “kvl” represents Kirchoff's Voltage Law, which law maybe used to relate the known quantities in the measurement circuit to theunknown quantities to be found.

While executing the algorithm needed to solve for the unknown bipolarand unipolar impedances and currents shown in FIG. 9, a microcontrollermay divert the constant current signals via the 2N switch array shown inFIGS. 11A and 11B as a 1:16 multiplexor to the appropriate catheterelectrode. The switch array may be kept at a manageable size byrequiring only 2 poles for N electrodes to solve for the respective 2Nunipolar and bipolar impedances.

The system 10 may also maximize catheter delivered current consistentwith applicable patient safety standards, minimize the risk ofinadvertent cardiac or nerve stimulation by elimination of unipolarcharge delivery, and deliver a multi-spectral measurement current thatallows for the simultaneous measurement and resolution of unipolar andbipolar impedances. Referring now to FIGS. 10 and 11, schematic views ofa corresponding circuit implementation are shown. The simultaneousexcitation of multi-frequency waveform for each electrode may beaccomplished using quantity M programmable and low-cost “chip” clockoscillators. The clock oscillators generate pulse waveforms that areapplied to their respective Gaussian band-pass filters, which in turnmay remove all artifacts except the fundamental sine waveform. By thenature of their dampened sinusoidal waveform outputs, Gaussian filteredsignals may contain negligible amounts of unipolar energy. For example,a short (or “runt”) pulse that would normally transfer unipolar energyif unfiltered may be removed to create a waveform with no evidence ofunipolar energy following operation by a third-order Gaussian band-passfilter. Typically, unipolar charge delivered to a heart chamber must beless than 50 nC (50e-9 Coulombs) to insure no risk of stimulation andcapture by the heart's atrium or ventricle. In FIGS. 11A and 11B, energyis transmitted in the high frequency (100 kHz) and low frequency (12.5kHz).

After Gaussian filtering, a Howland constant current amplifier may bemodified to enhance functionality. Once of these improvements may be inthe form of a passive summation array that is used to create a compositewaveform from individual sinusoids. The summation function may beaccomplished by using a passive weighting function, whereby a discretegain equal to: R₁/R_(filter,m) may be applied to each individualfrequency (monochrome) depending on the R_(filter,m) weighting values.This operation ensures that each frequency constituent current iscompliant with medical equipment safety standards, which stipulate thatcurrent delivered to patients via catheters 12 positioned in the heartmust not exceed limits depending on frequency. To comply with recentInternational Electrotechnical Commission (IEC) standards, currentdelivered to heart chambers at a frequency of 10 kHz must not exceed 100uA root mean square (rms), whereas current delivered to the heart at 100can be ten times greater, but cannot exceed 1 mA rms. The relation forconstant current may be calculated by:

$\begin{matrix}{\frac{R_{2} + R_{3}}{R_{4}} = \frac{R_{1}}{\left( {2R_{{filter},{m = 1}}} \right){{\left( {2R_{{filter},{m = 2}}} \right){}\left( {2R_{{filter},{m = 3}}} \right)}}\ldots}} & (18)\end{matrix}$

Once the individual monochromes are combined into one multi-frequencycomposite waveform, the modified Howland current amplifier may provide aconstant amplitude delivered patient current, regardless of tissuetermination impedance, as long as a coordinated selection of resistorsR₁₋₄ and R_(filter,m) obey the constant current relationship shown inFIG. 10.

The circuit shown in FIG. 10 may improve the traditional Howland circuitin two primary ways. First, a capacitor C₁ may be added as acompensation capacitor that ensures that op-amp U₁'s closed-loopresponse (the bandwidth over which it provides a constant currentfunction) remains stable and does not evidence peaking orself-sustaining oscillations. The value of C₁ may be selected to providea “flat” closed loop response slightly higher than the highest frequencymonochrome. As a non-limiting example, a value of 120 pF may be selectedto assure a smooth response to a frequency of 100 kHz, the highestfrequency monochrome in the composite waveform.

Second, a shunt path resistance R₅ may be added so that as the catheter12 traverses between very high and low conductivity tissue, the op-ampU₁ always remains in its linear operating region. The current divertedto this shunt may be very small, typically less than 1% of thepatient-delivered current, leaving the patient current constant within99% of the maximum delivered amount, regardless of catheter electrodetermination impedance. This function may be necessary to ensure that theop-amp does not saturate (or open circuit) and propagate a unipolar“glitch” to the patient.

The system 10 may further be used to measure and resolve unipolar andbipolar impedances simultaneously. Each of the N simultaneously measuredelectrodes 24 may be supported by a circuit to generate and measure theexcitation current. For this reason it may be desirable to limit N toless than the total number of electrodes by multiplexing the generationand measurement circuitry to multiple electrodes (for example, as shownin FIGS. 11A and 11B). In this way it may be possible to trade off themeasured speed achieved by simultaneously measuring electrodes foreconomy in circuit size and cost.

Each of the N generation circuits may create an excitation currentcomposed of the superposition of a constant-current phasor for everymeasurement frequency. For example, FIGS. 15 and 16 show non-limitingexamples of an excitation signal composed of 10 uA 12.5 kHz and 100 uA100 kHz signals in superposition. The excitation current may be passedthrough a sense resistor and out to the catheter electrodes 24, as shownin FIG. 8. Complex voltage at each frequency of interest may be measuredat the output of each constant-current driver, and across each senseresistor. Next, one or more of the current drivers may be deactivatedand the complex voltage measurement may be repeated. As shown in FIG. 9,the two sets of voltages may be sufficient to solve a system ofequations of all unipolar and bipolar impedances.

The system 10 may further be used to measure, detect, and resolvecatheter delivered multi-spectral, simultaneous measurement currentsinto their mono-frequency constituents. For example, it may be necessaryto resolve the superimposed measurement currents into mono-frequencyphasors before using them to determine impedance. As shown in FIG. 8,the voltage phasors V₁ . . . V_(N) and V_(R1) . . . V_(RN) may bedetermined by first taking a series of real voltage measurements acrossthe corresponding elements. These samples are taken so that severalperiods of the slowest frequency are captured. A discrete Fouriertransform may be applied to each series. A full transform may becomputed, or a partial transform may be applied by only computing theresults for the bins containing each excitation frequency. If a fulltransform is applied, the results for the excitation frequency bins maybe kept and the rest discarded. Once the transform is complete, thecomplex results for the bins containing each frequency of interest maybe taken to be the voltages V₁ . . . V_(N) and V_(R1) . . . V_(RN) ateach frequency and used to solve the impedance calculations. Forexample, FIG. 15 shows the spectral amplitude of the measurement signaldisplayed in FIG. 16.

The system 10 may further be used to disambiguate unipolar and bipolarimpedances from parasitic pathway impedances resulting from non-idealcatheter delivery wire impedance, inter-catheter wire field couplingmechanisms, and ablation delivery system signal splitter and filterconductive pathways and field coupling mechanisms. Parasitic pathwaysare inevitable in a distributed system containing an ablation generator52, interfacing equipment to ECG monitors, catheter wires that exhibitimpedance in the form of resistance and inductance (Z=r+Ai), andcapacitive coupling or shunting from one electrode's set of wires toanother set of wires connected to other electrodes. Without extraction,these pathways will corrupt measured signals and render inaccuratecatheter impedances.

These parasitic impedances (or admittances) may be solved for andde-embedded from the desired catheter electrode impedance by defining areference plane where the load is removed and left open (as shown inFIG. 12A). For example, this plane may be defined at the electrode planeof a catheter, such that all of the parasitic impedance effects could beaccounted for and removed from unipolar and bipolar tissue impedanceelements. For the two electrode case (for example, shown in FIG. 9),N=2, once the open circuit plane is defined, there may be two KVL loopequations with four parasitic elements: Z_(serp1,2) and Z_(shp1,2) asunknowns. The signal currents are applied, and the measurements may betaken at the voltage source nodes V₁ and V₂, as well as at the currentinference nodes V_(R1) and V_(R2). Next, a short circuit may be placedfrom E and E₂ (for example, as shown in FIG. 12B). The short circuit mayproduce a second condition that creates 3 additional KVL loop equations,and only one more unknown, i_(b,1). There are now five equations thatbalance the five unknowns. The parasitic impedance elements may then besolved, and their effect removed from catheter electrode impedancemeasurements.

The system 10 may further be used to adapt wide-ranging voltage andcurrent measurements resulting from similarly wide-ranging values ofblood, pre-ablation cardiac tissue, post-ablation cardiac tissue, and/orice formation on a cryoablation catheter. In order to maintain a highsignal-to-noise ratio (SNR) while allowing for a wide range of impedancemagnitudes, a variable amount of gain may be needed between each voltagenode (V₁ . . . V_(N) and V_(R1) . . . V_(RN) as shown in FIG. 8) and theADC that measures the real voltage at that node. If too little gain isapplied relative to the load impedance, and SNR, and thus themeasurement reliability, may suffer. Conversely, if there is too muchgain, the nonlinearities may be introduced as the signal is distorted orclips at the extremes of the ADC's measurement capabilities. To preventthis, each measurement may begin with a high amount of gain applied. Ifthe ADC measures a value that is outside of the linear region in thecenter of its measurement capabilities, then all samples may bediscarded, gain may be reduced, and measuring may be restarted. Thissequence may be repeated until gain is sufficiently low that no samplesare outside the linear region, at which point the sampled values may besaved and the impedance calculation may proceed. The gain adjustmentprocess is shown in FIG. 14.

A set of high precision resistive and reactive standards may beassembled upon the impedance meter apparatus printed circuit boardassembly, and then used as a special case measurement so that theimpedances derived can be referred to in terms of absolute physicalunits of resistive and reactive ohms.

Calibration may be required because individual amplifiers and filtersused in the impedance meter may produce non-ideal signal pathwaybandwidths and phase responses, and because sampling may produceartifacts. So that the impedance meter can render accurate results, themeter may be calibrated to offset and nullify these non-idealities.

Assuming that parasitic impedances are accounted for, a highly preciseset of resistive and reactive standards may be placed in positionsemulating the catheter electrode unipolar and bipolar positions. Thesestandards may then be referred to automatically via the switch array,and their values used as a reference by performing an impedance reading.While rendering a result, an internal microprocessor may compare themeasured reading to a priori values of the precision internal resistiveand reactive standards and correct the otherwise skewed measurementreading. The offset may then be recorded and applied to subsequentcatheter unipolar and bipolar measurements.

Since it may be impractical for a physician to remove a catheter from anablation generator, reconnect the catheter to a stand-alone impedancemeter, and then reconnect to the ablation generator, some means shouldbe provided to allow the impedance meter to remain connected to thegenerator 52 and the catheter 12. Yet, some means must also be providedthat isolates and protects the impedance meter from the high level ofradiofrequency energy (in cases wherein a radiofrequency generator isused) or high voltage (in cases wherein a pulsed field ablationgenerator is used) present on catheter wires that are simultaneouslyconnected to the generator 52, ECG monitor 56, and the impedance meter55.

A system 10 is shown in FIG. 17 that may isolate the impedance meter 55.During ablation, an inline PIN diode may be reverse biased to act as anopen switch, preventing radiofrequency or high voltage energy fromentering the impedance meter. During an impedance meter reading, theseries PIN diode may be forward biased and create a highly conductivepath to the catheter electrode wire, whereas the shunt PIN diode may bereversed biased and act as an open circuit.

A detector circuit may sense the application of ablation energy andinstantaneously control the application of voltages necessary tocorrectly bias the two PIN electrodes. The automatic circuit may negatethe need for communication of an on/off signal from the generator,allowing the impedance meter to be connected to a variety of ablationgenerators without concern of manufacture or revision of generatorhardware or software.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. In addition, unless mention was made above tothe contrary, it should be noted that all of the accompanying drawingsare not to scale. A variety of modifications and variations are possiblein light of the above teachings without departing from the scope andspirit of the invention, which is limited only by the following claims.

What is claimed is:
 1. A system for assessing electrode-tissue contactstatus, the system comprising: a medical device including a treatmentassembly having a plurality of electrodes; and a control unit incommunication with the medical device, the control unit programmed to:determine a difference value between a maximum impedance magnitude at alow frequency for one of a plurality of electrodes and a absoluteminimum impedance magnitude at the low frequency across all of theplurality of electrodes; determine a difference value between a maximumimpedance magnitude at a high frequency for the one of the plurality ofelectrodes and a absolute minimum impedance magnitude at the highfrequency across all of the plurality of electrodes; determine adifference value between a maximum impedance phase at the high frequencyfor the one of the plurality of electrodes and a absolute minimumimpedance phase at the high frequency kHz across all of the plurality ofelectrodes; correlate the difference values to each other for each ofthe plurality of electrodes; and assign to each of the plurality ofelectrodes a discrete prediction variable based on the correlations, theassigned discrete prediction variable indicating electrode-tissuecontact status.
 2. The system of claim 1, wherein the low frequency isbetween approximately 5 kHz and approximately 15 kHz.
 3. The system ofclaim 2, wherein the low frequency is 12.5 kHz.
 4. The system of claim 2wherein the high frequency is between approximately 80 kHz andapproximately 140 kHz.
 5. The system of claim 4, wherein the highfrequency is 100 kHz.
 6. The system of claim 4, wherein correlating thedifferences to each other for each of the plurality of electrodesincludes applying a linear model to the difference values.
 7. The systemof claim 6, wherein each of the plurality of electrodes is assigned adiscrete prediction variable that is either 0 or
 1. 8. The system ofclaim 7, wherein a discrete prediction variable of 0 is assigned to anelectrode when the electrode is not in contact with tissue and adiscrete prediction variable of 1 is assigned to an electrode when theelectrode is in contact with the tissue.
 9. The system of claim 8,wherein a discrete prediction value of 2 is assigned to an electrodewhen the electrode is in excessive contact with tissue.
 10. The systemof claim 8, further comprising delivering ablation energy to any of theplurality of electrodes to which a discrete prediction variable of 1 isassigned.
 11. A system for assessing electrode-tissue contact status,the system comprising: a medical device including a treatment assemblyhaving a plurality of electrodes; and a control unit in communicationwith the medical device, the control unit programmed to: record aimpedance magnitude value at a first frequency by each of a plurality ofelectrodes and determine a maximum impedance magnitude value of theplurality of impedance magnitude values at the first frequency; record aimpedance magnitude value at a second frequency by each of the pluralityof electrodes and determine a maximum impedance magnitude value of theplurality of impedance magnitude values at the second frequency; recorda impedance phase value at the second frequency by each of the pluralityof electrodes and determine a maximum impedance phase value of theplurality of impedance phase values; record a minimum impedancemagnitude value at the first frequency across all of the plurality ofelectrodes; record a minimum impedance magnitude value at the secondfrequency across all of the plurality of electrodes; record a minimumimpedance phase value at the second frequency across all of theplurality of electrodes; for each of the plurality of electrodes,calculate a first difference value between the maximum impedancemagnitude value and the minimum impedance magnitude value recorded atthe first frequency; for each of the plurality of electrodes, calculatea second difference value between the maximum impedance magnitude valueand the minimum impedance magnitude value recorded at the secondfrequency; for each of the plurality of electrodes, calculate a thirddifference value between the maximum impedance phase value and theminimum impedance phase value recorded at the second frequency; for eachof the plurality of electrodes, apply a linear model to all of thefirst, second, and third difference values to determine a continuousprediction value for each of the plurality of electrodes; and a displayin communication with the control unit.
 12. The system of claim 11,wherein a continuous prediction value for each of the plurality ofelectrodes is determined using the equation:continuous prediction value=a*1+b*ΔZ ₁₀₀ +c*ΔZ _(12.5) +d*ΔØ ₁₀₀. 13.The system of claim 11, the control unit being further programmed to:assign a discrete prediction value to each of the plurality ofelectrodes based on a corresponding continuous prediction value, thedisplay configured to display at least one of the continuous predictionvalue for each of the plurality of electrodes and the discreteprediction value for each of the plurality of electrodes.
 14. The systemof claim 13, wherein the at least one of the continuous prediction valueand the discrete prediction value is displayed in at least one of text,color, and graphical formats.
 15. The system of claim 14, wherein agraphical representation of the plurality of electrodes is displayed,each of the plurality of electrodes being displayed as a color thatcorresponds to at least one of the continuous prediction value and thediscrete prediction value, the colors changing in real time with the atleast one continuous prediction value and discrete prediction value. 16.The system of claim 11, wherein the second frequency is higher than thefirst frequency.
 17. The system of claim 16, wherein the first frequencyis between approximately 5 kHz and approximately 15 kHz.
 18. The systemof claim 17, wherein the first frequency is 12.5 kHz.
 19. The system ofclaim 16, wherein the second frequency is between approximately 80 kHzand approximately 140 kHz.
 20. The system of claim 19, wherein thesecond frequency is 100 kHz.
 21. The system of claim 13, wherein thecontinuous prediction value is a number between 0 and
 1. 22. The systemof claim 21, wherein a discrete prediction value of 0 is assigned to acontinuous prediction value that is less than a cutoff value and adiscrete prediction value of 1 is assigned to a continuous predictionvalue that is greater than or equal to the cutoff value.
 23. The systemof claim 22, wherein the cutoff value is 0.4.
 24. The system of claim22, wherein the cutoff value is 0.5.
 25. The system of claim 13, whereinthe continuous prediction value is a number equal to or greater than 0.26. The system of claim 25, wherein a discrete prediction value of 0 isassigned to a continuous prediction value that is less than a firstcutoff value, a discrete prediction value of 1 is assigned to acontinuous prediction value that is greater than or equal to the firstcutoff value and less than a second cutoff value, and a discreteprediction value of 2 is assigned to a continuous prediction value thatis greater than or equal to the second cutoff value.
 27. The system ofclaim 26, wherein the first cutoff value is 0.4 and the second cutoffvalue is 1.5.
 28. A system for displaying electrode-tissue contactstatus, the system comprising: a medical device including a treatmentassembly having a plurality of electrodes; and a control unit incommunication with the medical device, the control unit programmed to:record a first maximum impedance magnitude value from each of aplurality of electrodes at 12.5 kHz; record a second maximum impedancemagnitude value from each of the plurality of electrodes at 100 kHz;record a maximum impedance phase value from each of the plurality ofelectrodes at 100 kHz; record a first minimum impedance magnitude valueacross all electrodes of the plurality of electrodes at 12.5 kHz; recorda second minimum impedance magnitude value across all electrodes of theplurality of electrodes at 100 kHz; record a minimum impedance phasevalue across all electrodes of the plurality of electrodes at 100 kHz;determine a difference value between the first maximum impedancemagnitude value for each of the plurality of electrodes and the firstminimum impedance magnitude value; determine a difference value betweenthe second maximum impedance magnitude value for each of the pluralityof electrodes and the second minimum impedance magnitude value;determine a difference value between the maximum impedance phase valuefor each of the plurality of electrodes and the minimum impedance phasevalue; apply a linear model to the difference values to determine acontinuous prediction value between 0 and 1 for each of the plurality ofelectrodes; and assign a discrete prediction value of 0 to each of theplurality of electrodes for which the continuous prediction value isless than a cutoff value and assign a discrete prediction value of 1 toeach of the plurality of electrodes for which the continuous predictionvalue is equal to or greater than the cutoff value; and a display incommunication with the control unit, the display configured to displayat least one of the continuous prediction value for each of theplurality of electrodes and the discrete prediction value for each ofthe plurality of electrodes in at least one of text, color, andgraphical formats.